This also moves `__all` from `sfinae_helpers.h` to `conjunction.h`.
This brings the code base closer to having only a single style of `enable_if`s.
This patch adds std::experimental::observer_ptr (n4282) and also fixes LWG2516. Co-Authored-By: Louis Dionne <ldionne.2@gmail.com> Differential Revision: https://reviews.llvm.org/D63230